Acoustic logging tools for measuring various formation properties adjacent a borehole are well known. Measurement of subsurface geological formations may be taken by wireline logs after the well has been drilled or by MWD tools during the drilling of a well, sometimes referred to as an earth borehole. In either application, bulk porosity and elastic properties of the formation may be derived by measuring formation resistivity, neutron, gamma ray and sonic velocities. Important information taken from sonic logs may be used in a variety of applications, including correlation between wells, seismic plots, porosity determination, lithology determination, detection of overpressured formation zones, and the conversion of a seismic time trace to an indication of varying lithology based upon the velocity of sonic waves in the earth formation.
Typically, in a drillstrng used in drilling an earth borehole, an MWD acoustic or sonic logging tool includes a sonic source positioned at one location along the drill collar to transmit to one or more locations positioned along the drill collar. The sonic source emits acoustic energy that propagates into the formation and which also travels along the borehole through the borehole fluid. Receivers in the drill collar detect the acoustic energy which comes from the formation to determine the acoustic velocity in the formation. Normally, both the shear wave velocity and the compressional wave velocity in the formation are greater than the velocity of acoustic waves in the borehole fluid. The difference in propagation times of the various acoustic signals allows their separation and measurement. For example, a typical prior art acoustic signal is generated to propagate through the borehole fluid and the formation, as well as along the logging tool body itself, to at least one receiving transducer. Depending upon the various propagation effects of the acoustic signal traveling along the tool body, and through the borehole and the formation, two or more receiving transducers may be necessary. The time it takes for the acoustic energy to propagate from the transmitter to a receiver, or from one receiver to another receiver, is then measured. Since the commence time of the transmission is known and the time of the first arrival of acoustic energy at each receiver after having passed through the formation is measured, the propagation time of the signal through the formation, referred to as interval transit time,” is thus determined. Variations in interval transit time, either between the transmitter and one or more receivers, or between receivers in one or more receiver pairs are thus measured and calculated in a conventional sonic Delta-T Sensor as part of the MWD tool, “T” being an abbreviation for time. A Sonic Delta-T sensor, like many other sensors, is simply an array of sensors designed to measure borehole or formation properties. Although sonic logs provide valuable information for various formation properties, certain disadvantages exist in either application due to “propagation effects” of the transmitted acoustic signals through the formation and the borehole fluid, as well as along the drill collar.
During drilling of the well a number of different paths are available for acoustic energy propagation. Energy propagating along one path travels through the earth formation surrounding the borehole, while energy propagating along another path travels through the annular space between the borehole and the exterior surface of the drill collar, normally filled with drilling fluid. Energy propagating along another path travels directly through the drill collar at an acoustic velocity typically greater than the acoustic velocity of the formation and which may reverberate through the drill collar housing the acoustic logging sensors and associated circuitry.
Depending upon the path or paths along which the acoustic energy propagates, the acoustic energy may propagate as certain wave fields or modes which include: (1) compressional, torsional and lateral waves that propagate along and through the tool itself; (2) compressional waves that propagate within the borehole; (3) compressional and shear waves that propagate through the formation; and (4) leaky-p, tube and pseudo-Rayleigh waves that propagate along the interface between the formation and borehole.
In borehole logging, studies of the different acoustic modes propagating in the formation or at the formation/borehole interface provide information about the elastic contents of the formation, rock texture, porosity, fluid content, rock fracturing, and other data. For quantitative analysis, it is necessary to isolate the various waveform modes. Consequently, conventional logging tools are designed to suppress undesired waveforms (noise) either by structural design or by post-processing software.
It is normally desired that the detected signal remain free of any acoustic energy emitted by the transmitter into the tool body and propagating along and through the tool body to the one or more receivers. Since the speed of sound in the borehole or in the tool body is usually faster than that of the formation, the tool signal and the borehole signal arrivals in such cases usually occur before the formation signal arrives. Since the sonic logging device merely records signals as they are received, it is difficult to distinguish whether a signal has traversed the borehole, the formation or the tool body. Thus, such first arriving signals propagating through the borehole and/or the tool body may falsely be assumed to be first arriving signals propagating through the formation.
Although various solutions have been developed in the wireline logging industry for attenuating or suppressing the propagated tool signal so that formation arrival may be detected without interference from the propagated tool signal, such solutions are generally limited to an alteration of the structural design of the wireline tool which are not feasible for use on MWD tools. For example, since wireline tools arc not required to function as a load bearing member, it is possible to form an array of staggered openings along the length of a sidewall of the wireline tool's housing. The openings serve to lengthen the total path that a housing propagated acoustic signal must follow. As a result, the signal is not only delayed, but is also attenuated due to the increased path length and additional openings that the scattered signal must traverse.
In MWD applications, however, the acoustic tool is incorporated into a drill collar which must be able to withstand the immense forces and accelerations encountered during the drilling of the well. Numerous holes or indentations in the sidewalls of the drill collar would weaken the collar so that it would not be able to withstand normal wear and tear during drilling operations. Moreover, the fluid isolation between the inside of the drill collar and its exterior would be lost or reduced. Additionally, conventional convoluting of the sidewall of an acoustic tool so that the tool takes on a uniform thickness, yet tortuous longitudinal cross-section, is equally problematic in that such shapes either are too weak or require an impractically large portion of the limited diameter of the tool.
One attempt to resolve the propagation of an acoustic signal traveling through a wireline tool body is found in U.S. Pat. No. 5,036,945 to Hoyle et al. Hoyle et al disclose a sonic well tool having a first and second attenuation and delay apparatus for attenuating and delaying the signal traversing the tool body. The first attenuation and delay apparatus includes interleaved rubber and metal-like washers for attenuating compressional and flexural waves propagating along the tool body, and a bellows section having a corrugated shape and a thin traversed dimension. The second attenuation and delay apparatus includes a mass loading ring surrounding the housing of the well tool, and a bellows section having a corrugated shape and a thin traverse dimension.
U.S. Pat. No. 5,229,553 to Lester et al discloses an acoustic isolator for use with a well logging tool having transducers in a first and third tool segment which are to be acoustically isolated from receivers in a second and fourth tool segment. The acoustic isolator consists of vertebrate links of spools encased by resilient boots which are arranged end to end in tandem configuration. A plurality of split shells interconnect the spools by externally gripping the boots covering the end portions of the respective adjacent spools.
U.S. Pat. No. 4,872,526 to Wignall et al, deals with a wireline logging tool that utilizes various mechanical means for isolating the transmitter from the receiver.
Since most solutions to resolving the “propagation effects” of an acoustic signal traveling through a wireline tool body are impractical for use on MWD tools, and it is preferable to measure characteristics of the formation adjacent the well bore in a timely fashion while drilling, a number of different attempts have been made to remove unwanted acoustic noise as a result of the “propagation effects” of a transmitted acoustic signal using an MWD tool.
For example, U.S. Pat. No. 5,467,320 to Maki, Jr. discloses an acoustic formation apparatus having first and second receivers with a common azimuth spaced vertically along the drill stem and connected, respectively, to first and second amplifiers, first and second band pass filters, first and second clipping circuits, a single delay circuit and a cross-correlating circuit. The signals are received, filtered, amplified, clipped, delayed, and then cross-correlated utilizing variable time delays. The resulting cross-correlation function is analyzed using a microprocessor which removes stationary correlation peaks and ignores very short correlation delay times. This enables removal of the substantially fixed aspect of the drill pipe transmitted signal and the very fast propagating waves so that the processed cross-correlation function output is substantially related to the formations and the larger, overpowering signal transmitted along and through the drill collars is substantially avoided.
Another example is U.S. Pat. No. 5,357,481 to Lester et al which discloses an MWD borehole logging tool having a sonde constructed of a plurality of segments that are axially rotatable with respect to each other. Each one of two of the segments includes a compartment in which is mounted a dipoled bender bar transmitting transducer. Two additional segments each contain one or more binaurally sensitive receiver transducers. Monopole transmitting and receiving transducers are also included in the respective appropriate segments. An acoustic isolator separates the transmitting transducers from the receiving transducers. The borehole logging tool is said to be capable of reducing noise during the acoustic logging process such as: (1) random noise by use of multiple receivers to make redundant measurements, and (2) phasing the receivers to exclude noise from certain directions.
U.S. Pat. No. 3,982,606 to Drumheller et at deals with the suppression of acoustic noise from a transmitter, through a borehole, to a receiver, utilizing mechanical means to obstruct the travel path of unwanted acoustic signals traveling within the borehole fluid between the transmitter and the receiver. Thus, the obstructed path will delay the arrival time of the unwanted acoustic signals at the receiver.
U.S. Pat. No. 5,475,731 to Rasmusson discloses an echo canceling system and method in a cellular telephone system using an echo estimate to modify an error signal which is obtained by the difference between an echo signal and the echo estimate. The modified error signal, rather than the error signal itself, is transmitted. By using the echo estimate as a parameter for modifying the error signal improved hands free performance may be obtained in a cellular telephone. The echo estimate signal is transmitted with the original signal in order to cancel the echo error when it is received.
U.S. Pat. Nos. 5,418,335 and 5,371,330 to Winbow disclose first and second end acoustic sources and an intermediate acoustic source positioned to enhance the signal to noise ratio and substantially eliminate tube wave interference. Each first end and second end acoustic source create a partial acoustic pressure null proximate the first and second longitudinal ends of the apparatus, thereby preventing at least a portion of the intermediate source acoustic pressure waves from propagating through the borehole beyond the longitudinal ends of the apparatus. Consequently, tube wave dominance in the regions above and below the apparatus is minimized by the creation of the near zero acoustic pressure condition around and beyond each end of the apparatus using the first end and second end acoustic sources to block the tube wave affect of the intermediate acoustic source from propagating through the borehole above and below the apparatus.
U.S. Pat. No. 5,309,404 to Kostek et al pertains to reducing the effects of quadrupole and higher order moments of acoustic signals transmitted from a transversely mounted acoustic source in a MWD tool by angular displacement of the receiver such that the quadrupole mode of the acoustic signal is rendered insignificant. In another embodiment, a plurality of receivers are uniquely positioned to eliminate the quadrupole mode by timing each receives individual signal to obtain a resulting signal by the summation of the individual signals and averaging the same to eliminate the quadrupole mode.
U.S. Pat. No. 5,274,606 to Drumheller et al presents an electronic circuit for digitally processing analog electrical signals produced by at least one acoustic transducer. Preconditioned data is used to counteract distortions caused by the drill string, the distortion corresponding to the effects of multiple pass bands and stop bands having characteristics dependent upon the properties of the drill string. The preconditioned data is transmitted to the first end of the drill string and detected at a second end of the drill string.
U.S. Pat. No. 4,796,237 to Hutchens et al is directed to determining the presence of defects in cement surrounding a casing set within a well bore by using the amplitude of the casing reverberation noise from a known signal source such as a transmitter, and the time between the transmission of the known signal and the reception of the first casing reverberation noise. This amplitude and delay time are then used to eliminate the casing reverberation noise by either transmitting a series of negative signals having amplitudes equal to the immediately preceding received acoustic reverberation at a time later than the positive signal so that the acoustic reverberations are driven to a null, or scaling the amplitude of the known signal delayed a time between the transmission of the known signal and the reception of the signal, and subtracting the scaled signal from the stored known signal to generate or refine a signal from which casing reverberation noise has been removed. In either method, the noise signal is known and since the transmitter abuts the casing adjacent the surrounding cement, propagation effects due to other borehole noise are not considered.
U.S. Pat. No. 4,590,593 to Rodney deals with an electronic noise filtration system for use in improving the signal to noise ratio of acoustic data transmitted from a downhole transducer in a MWD system. A pair of acoustic receiving transducers are spaced apart on a given flow path for generating a respective output signal responsive to acoustic pulses propagating through the drilling fluid. The difference in output signals of the transducers is determined by selectively delaying one of the output signals to a difference determining means and which is then compared to the difference during the absence of purposely-generated downhole data to effectively minimize the difference and eliminate acoustic noise in the flow path.
U.S. Pat. No. 4,153,815 to Chaplin et al discloses a method of reducing the amplitude of vibrations received at a selected location from a source of reoccurring noise by feeding to the location a specifically generated secondary vibration which at least partially nulls the vibrations from the noise source at the selected location. A triggering signal derived from the selected source is used to synchronize the generation of the secondary vibration with that of the vibration to be at least partially nulled. Since the noise source is reoccurring, the amplitude of the vibrations is known and the cancellation signal may be easily simulated. However, the process relies solely on the derivation of a cancellation signal from a known source. Noise originating from unknown sources cannot easily be estimated and/or eliminated. Moreover, the process does not provide for distinguishing between a preferred signal hidden within an unknown noise signal that arrives simultaneously at a receiver as noise.
U.S. Pat. No. 4,215,425 to Waggener relates to a communication system and apparatus for receiving and interpreting data signals being telemetered to the surface of the earth in an MWD system. A filter is provided for use and detection in a phase shift keying transmission system of the type where modulation is achieved by temporary inter-directional modification of a carrier frequency.
U.S. Pat. No. 5,510,582 to Birchak et al provides an apparatus for sonic well logging having at least one transmitter and at least one receiver. Positioned between the transmitter and receiver is an acoustical attenuation section intended to attenuate sonic waves traversing the sonic logging tool. This acoustical attenuation section generally includes one or more fluid filled cavities in the sonic logging tool, into which are inserted inertial masses. The cavities are generally shaped to receive the attenuators and are slightly larger so that a gap will exist between the walls of the cavities and the inertial masses (attenuators) as the attenuators are positioned in the cavity. Therefore, the tool body, the fluid and the inertial masses are designed to act together to mechanically attenuate undesired acoustic signal propagation.
British Patent Number GB 2,266,372 A to Scherbatskoy discloses a sonic logging arrangement having an MWD telemetry system in which a drilling fluid pump at the earth's surface circulates drilling fluid through a drill string and produces a high differential pressure between the inside and outside of the string, thereby producing acoustic pressure waves in the formation surrounding the borehole.
British Patent Number 2,300,048 A and U.S. Pat. No. 5,639,997 to Mallett is directed to an acoustic noise canceling apparatus for well logging that is said to cancel interfering signals propagating through the tool body by transmitting an acoustic signal into the formation that also traverses the tool body for receipt by two receivers. The first receiver is isolated, receiving only the acoustic signal propagating through the tool body, and the second receiver receives acoustic signals traversing both the formation and the tool body. A processor then subtracts the tool signal of the first receiver from the combined tool and formation signal of the second receiver resulting in a single formation signal. The apparatus is restricted to canceling noise from a known signal such as a transmitter, and does not take into account propagation effects resulting from the signal passing through the borehole fluid and/or the formation. Moreover, the apparatus is limited to placing each isolated first receiver immediately adjacent each second receiver precluding the ability to consider propagation effects of multiple noise signals propagating from above and below the tool body.
Other publications of general interest describing mechanical means for suppressing noise interference and signal processing of sonic arrays in MWD tools include:    J. Aron, S. K. Chain, R Dworak, K. Hsu, T. Lau, J-P. Masson, J. Mayes, G. McDaniel, C. Randall, S. Kostek, T. J. Plona, Sonic Compressional Measurements While Drilling, SPWLA 35th Annual Logging Symposium, Jun. 19-22, 1994, paper SS.    John Minear, Robert Birchak, Carl Robbins, Eugene Linysev, Bruce Mackie, David Young and Robert Malloy, Compressional Slowness Measurements While Drilling, SPWLA 36th Annual Logging Symposium, Jun. 26-29, 1995, paper VV.    Christopher V. Kimball, Thomas L. Marzetta, Semblance processing of borehole acoustic array data, Geophysics, Vol. 49, No. 3, March, 1984, pp 274-281.    A. L. Krkijan, S. W. Lang, and K. Hsu, Slowness estimation from sonic logging waveforms, Geoexploraton, Vol. 27, 1991, pp 215-256.    C. F. Morris, T. M Little, and W. Letton, III, A New Sonic Array Tool for Full Waveform Logging, 59th Annual Technical Conference and Exhibition, Sep. 16-19, 1984, paper SPE 13285.
In view of the prior art and its limitations, a specific need exists to provide an apparatus and method for determining a preferred acoustic signal indicative of borehole and/or formation characteristics by eliminating acoustic noise signals that may propagate through the drill collar or tool body, as well as the formation, borehole and borehole/formation interface as acoustic measurements are taken using an MWD tool.